A trend in optical communication systems is to integrate functions traditionally performed by discrete optical components onto an integrated optic ("IO") device. These functions include polarizers, filters, modulators, directional couplers, waveguides, etc. The IO device is typically fabricated from a substrate of lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3) and has a waveguide formed on a major surface. Common waveguides include the Mach-Zehnder and Balanced-Bridge interferometers. In the Mach-Zehnder, an input optical signal (e.g., from a laser) is split at a Y-junction in the waveguide into two equal components. "Mach-Zehnder" refers to the waveguide Y-junctions and arms, along with the modulators. Each component travels in a corresponding arm of the waveguide before being recombined at a second Y-junction.
To create an optical modulator using the Mach-Zehnder waveguide, electrically conductive electrodes are formed on the waveguide surface in the vicinity of the waveguide arms. A time varying voltage applied to the electrodes produces an electric field in the IO device substrate which changes the relative indices of refraction and therefore the optical path lengths of the waveguide arms. Modulation of the optical input signal occurs due to the fact that the relative phase of the optical signals in the arms varies according to the instantaneous amplitude of the time varying electrical signal driving the electrodes. The varying phase results in varying intensity of the optical signal at the interferometer output.
Since the modulator operates on light interference principles, its transfer function when properly biased is a sine function which imposes odd harmonic distortion to applied signals. In order to avoid generating second harmonics on the modulator output signal, a DC voltage bias signal is applied to the modulator to maintain the operating point about the half-power point of the modulator. The half-power point is the point on the transfer function sine curve where the slope (i.e., optical throughput/voltage) is a maximum. This corresponds to a 90.degree. relative phase shift between the two arms of the modulator. FIG. 1 is graphical illustration of the modulator's sine wave transfer function. Optical throughput is plotted along the vertical axis and bias voltage is plotted along the horizontal. The half power points are as illustrated as V.sub..pi./2 and -V.sub..pi./2 since the modulator can operate on either the positive or negative slope of the sine wave. In general, the modulator can be biased at any odd multiple of V.sub..pi./2 or -V.sub..pi./2 for proper operation.
The modulator typically receives an electrical data carrying input signal which modulates the light wave within the modulator to transfer the data content in the electrical signal to the lightwave. However, problems arise when the modulator is not properly biased to operate at its half-power point and as a result undesirable even harmonics are coupled into the output signal of the modulator. Therefore, closed loop bias control is often employed to ensure the modulator maintains its operating point within the linear region about the half-power point. Even harmonic distortion arises when the modulator is not operating at its half-power point which is a well known problem in the field of multichannel optical communication systems, such as a system for cable-television ("CATV") which utilizes the modulator to convert a data carrying radio frequency ("RF") signal to an optical data carrying signal.
A CATV system may have 75 channels operating at different carrier frequencies all multiplexed into a single signal for transmission within a frequency range of 50-500 MHz. This large number of closely-spaced carriers places strict requirements on the linearity of system elements such as the modulator in order to reduce the undesired harmonic distortions.
Conventional closed loop bias control systems superimpose an AC signal test tone (e.g., a sine wave) on the DC bias voltage value applied to the bias electrodes of the modulator. If the modulator is not operating at its half-power point, second harmonics of the AC signal test tone are created within the frequency spectrum of the modulator output signal. The magnitude of the second harmonic of the AC signal is indicative of the error between the desired DC bias signal value and the actual DC bias signal value. As an example, if there is no second harmonic energy within the frequency spectrum of the output signal, the modulator is currently operating at its half-power point and therefore no bias error exists. However, if second harmonic energy is present, a bias error exists and the bias signal value is adjusted to null the bias error.
A problem plaguing bias control systems are errors due to the limited dynamic range of known detection systems. This problem arises due to the fact that as steady state half-power point operation is approached, the magnitude of the second harmonic energy drops off very quickly while the fundamental remains relatively unchanged. Amplification and phase sensitive demodulation of the control signals is difficult when the signal of interest (namely the second harmonic) co-exists with a sizeable fundamental frequency component. Typically the relative size of the fundamental frequency component can be reduced somewhat through the use of well known filtering techniques. However, the implementation of these filtering techniques typically requires a fourth or fifth order filter to provide the amount of attenuation necessary for high accuracy bias control. Filters of such a high order are often cumbersome and impractical.